Importance of Brassica soil amendments for managing soilborne disease in ornamentals and vegetables

Final Report for GS07-061

Project Type: Graduate Student
Funds awarded in 2007: $9,944.00
Projected End Date: 12/31/2008
Grant Recipient: University of Arkansas
Region: Southern
State: Arkansas
Graduate Student:
Major Professor:
Craig S. Rothrock
University of Arkansas, Plant Pathology
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Project Information

Summary:

Soilborne pathogens of ornamental and vegetable crops cause production losses every year. However, current disease control methods offer few options for control and chemical fumigation is increasingly limited and costly. Additionally, reliable and sustainable disease management methods appropriate for limited resource farmers should be further researched due to the lack of available effective sustainable management methods. Brassicas, including mustards and canolas, contain secondary metabolites, including glucosinolates, which decompose into biocidal and volatile isothiocyanates, as well as other chemicals, which are inhibitory to plant pathogens. Thus, Brassica amendments appear to be a viable alternative to chemical soil fumigation practices for managing pathogen populations, including fungi and nematodes, as part of a sustainable agricultural system in the southern United States. However, additional information is needed on effective Brassica spp. and rates of incorporation. Objectives in the study were to determine the importance of Brassica cover crop selection and biomass application rates for disease management of the root-knot nematode, Meloidogyne incognita, and Rhizoctonia solani AG4. For petunia and impatiens experiments over two years, Brassica amendments significantly reduce disease symptoms and recovery of Rhizoctonia solani from plants. Brassica crops were not significantly different from each other. Rate of Brassica application had the greatest impact on disease symptom reduction and reducing Rhizoctonia solani isolation, with a rate of 3000 g/square m being more effective than lower rates. There was no significant interaction between cover crop and amendment rate. No phytotoxic effects from the Brassica crops were observed at any rate of application. Results from Meloidogyne incognita experiments with cucumber were not conclusive because of problems with infestation methods allowing survival of nematode inoculum during the decomposition period of the green manure, while exposing the nematode to the effects of the Brassica amendment. However, galling from the root-knot nematode has been significantly reduced on cotton using Fumus to levels similar to the fumigant Telone II. This study indicates that Brassica selection has less effect than the amount of Brassica amendment incorporated. Thus, selection should be based on biomass production and not glucosinolate levels produced by a cultivar. This holds great promise, as growers can make selections based on adaptation of the Brassica species and cultivar. Also, this should reduce costs by allowing growers flexibility by purchasing the lowest priced cultivars. The study did document the suppression of the fungus Rhizoctonia solani on ornamentals and this should apply to vegetable crops where Rhizoctonia solani is an important pathogen. Effective management of soilborne pathogens for vegetable and ornamental crops using Brassica amendments in both limited-resource farming operations and home production appears promising.

Introduction

Soilborne pathogens of ornamental and vegetable crops cause production losses every year. However, current disease control methods offer few strategies for control and chemical fumigation options are increasingly limited and costly. Additionally, there is currently a lack of environmentally responsible and affordable disease management methods available for limited resource producers. Brassicas, including mustards and canolas, contain secondary metabolites, including glucosinolates, which decompose into biocidal and volatile isothiocyanates, as well as other chemicals, which are inhibitory to plant pathogens. Managing pathogen populations, including fungi and nematodes, with Brassica winter cover crops appears to be a viable alternative to chemical soil fumigation as part of a sustainable agricultural system in the climate of the mid-south. However, additional information is needed on effective Brassica spp. and rates of incorporation.

Various research has investigated several Brassica crops for effectiveness of disease suppression, though these studies typically only used a Brassica crop that was not designed for biofumigation purposes. Few studies have addressed the use of Brassicas specifically intended for biofumigation efforts, and those studies typically focused on row-crop production. These experiments will address appropriateness and effectiveness of three Brassica winter cover crops including one specifically bred for higher levels of glucosinolates, as well as rates of amendment. The purpose of this project is to determine the appropriate Brassica species and cultivar and amendment rate for management of nematodes and Rhizoctonia solani on vegetable and ornamental crops. The root-knot nematode and R. solani were selected for assessment because of the significance of these pathogens in all levels of vegetable and ornamental production, including limited resource farmers and organic producers. The Brassicas used included the Indian mustard (B. juncea cv. Fumus), the Indian mustard Bionute, and a canola (Brassica napas cv. Jetton).

Growing Brassica biomass in the location needing treatment also can assist in more efficient resource management when compared to strategies that use seed meal hauled into a location and perhaps also improving effects of the green manure crop due to the high volatility of the active compounds. This could allow many small operations to afford effective and environmentally responsible disease control, while improving yields and crop value. Additionally, this project could improve sustainable agriculture efforts through the reduction of chemical inputs in local ecological systems, reductions in fuel consumption, and improved availability of affordable, simple disease management methods to limited resource and home growers.

The anticipated impact of this project is to develop recommendations for the management of soilborne diseases in limited resource farming operations. This would allow for the integration of an improved sustainable method of disease management into the existing agricultural systems.

Project Objectives:

1. To determine the importance of Brassica cover crop selection and biomass application rates in relation to disease management of root-knot nematodes (Meloidogyne incognita) affecting cucumber.

2. To determine the importance of Brassica cover crop selection and biomass application rates in the disease management of Rhizoctonia solani AG4 on petunias and impatiens

Cooperators

Click linked name(s) to expand
  • Craig Rothrock

Research

Materials and methods:

The experiments were conducted in microplots, allowing for greater control of operations than the field settings and the ability to artificially infest soils with an individual pathogen. Three experiments evaluated disease control using different species of Brassica and different application rates of these Brassicas on selected ornamental and vegetable crops. The experiments employed common ornamentals and vegetables crops in combination with disease management challenges commonly faced by home gardeners and small-scale producers. The selected ornamentals were petunia and impatiens and evaluated the management of Rhizoctonia solani AG4. The selected vegetable crop was a bush-type cucumber, Bushmaster in 2007 and Burpee Bush Champion in 2008, and evaluated the management of Meloidogyne incognita, the southern root-knot nematode.

Rhizoctonia solani inoculum was prepared as 10 to 14-day-old colonized millet seed. Inoculum was applied at a rate of 25 millet seed per 100 grams of soil. Meloidogyne.incognita inoculum was produced in the greenhouse on Rutgers tomatoes. The root-knot nematode inoculum consisted of soil and heavily infested root tissue chopped into 1-3 cm pieces and thoroughly mixed into the soil at an inoculation rate of 10 eggs and J-2 stage nematodes per cc of soil in 2007 and in 2008 total tomato root systems were placed in microplots for a target infestation rate of 15 eggs and J2’s per cc of soil. All inoculum was incorporated to a depth of 15 cm.

Brassica amendment treatments included using biomass of B. juncea cv. Fumus, specifically bred for high levels of glucosinolates, B. napus cv. Jetton, and B. juncea cv. Bionute. Rates were 750 g (1/2x rate), 1500 g (1x rate), and 3000 g (2x rate) of above ground Brassica biomass per square meter. In the field 1500 g/square m of above ground biomass is frequently produced by broadcasting seed of Fumus in late September to early October and allowing the crop to grow until early March, flowering. A negative control, pathogen infestation and no Brassica treatment, was included in each repetition. Five repetitions of each crop were performed. Additionally, two repetitions of all Brassica treatments and no pathogen infestation were performed for each ornamental and cucumber as a control to examine amendment phytotoxic or fertility effects. Brassica plants were dug and transported back from the field locations to local facilities and kept in a cooler at 4º C for no longer than 3 days prior to incorporation. Relative amounts of root mass compared to above ground tissue were calculated so that whole plants amendment levels were based on field results for above ground biomass. Biomass was cut into small sections, 3-6 cm long, using hand held clippers. This was done just before incorporation to prevent loss of volatile compounds. After chopping, the biomass was incorporated to a depth of 15 cm in each microplot. After incorporation of inoculum and biomass, plots were watered equivalent to 2.5 cm of rainfall.

After approximately 4 weeks post incorporation, decomposition period, ten young plants of each ornamental or 15 cucumber seed were planted in respective plots. After planting, all plots were thoroughly watered. Throughout the season, plants were periodically fertilized with MiracleGro water soluble fertilizer. All plants were checked and watered regularly. A temperature and moisture sensor was placed in all experiments. Upon culmination of the experiment, all plants were removed for analysis, including root tissue. The roots were weighed and the top tissue oven dried, weighed and averaged, on a per plant basis.

Disease caused by Rhizoctonia solani was measured on the ornamentals for all plants. Root discoloration was rated on a scale of 0 = 0%, 1=1-20%, 2=21-40%, 3=41-60%, 4=61-80%, 5=81-100%. Crown disease was rated on a qualitative scale as follows: 1= healthy, 2= pinpoint lesion, 3= lesion, 4= large or several lesions, 5= girdling lesions.
All ornamentals were gently washed free of soil and the crown and root sections plated on Ko and Hora selective medium for Rhizoctonia solani after plants were rinsed in water for 20 minutes and disinfested in a 0.5% solution of NaClO for 1.5 minutes. Twenty-four to forty-eight hours after plating, plates were examined for suspected Rhizoctonia solani colonies for transfer to potato dextrose agar for identification. Meloidogyne incognita damage on cucumber was assessed as a root gall rating for each plant. Gall ratings were based on amount of root tissue showing galling: 0=0%, 1=1-10%, 2=11-25%, 3=26-50%, 4=51-75%, 5=76-100%. However in 2008, due to excessive levels of galling, a different protocol was followed. This protocol involved collecting, washing, and photographing cucumber roots for later examination. Roots were then weighed, thoroughly chopped and reproduction of the nematode determined. Photographs were analyzed using ASSESS software (APS press). Individual images were calibrated and the following data were collected: average total root area, average total tap root length, average total root length (including main lateral roots and tap roots), and a ratio of average total area to average total root length.

Research results and discussion:

Results from Meloidogyne incognita infested cucumber plots were not conclusive. In 2007, disease pressure was lacking and in 2008 the cucumber crop was overwhelmed with extremely high disease pressure. For 2008 when severe galling was observed, there was a trend for root area to be less for the no Brassica treatment and for root area and average total root length to be greater for the highest rate of amendment compared to lower rates. More work needs to examine methods of infestation to simulate field conditions that allow for survival of the nematode inoculum during the decomposition period of the green manure, while exposing the nematode to the effects of the Brassica amendment.

In both the petunia and impatiens experiments over both years, all Brassica amendments reduced disease symptoms and lowered the percentage of plants where Rhizoctonia was recovered. Brassica crops were not significantly different from each other in disease symptom reduction or isolation. There was no significant interaction between cover crop and rate indicating that the various Brassica have similar effects. To show the effects of Brassica crop and rate of amendment on disease, data for impatiens are given and were similar for petunias. For impatiens, Brassica amendment reduced crown ratings, percentage of plants with crown lesions; Bionute 36%, Fumus 38%, and Jetton 36%, compared to the no Brassica treatment, 74%, over both years. Similarly the amendments reduced percent root discoloration; Bionute 22%, Fumus 24%, and Jetton 25%, compared to the no Brassica treatment, 34%, over both years. Isolation of Rhizoctonia solani from plants was also reduced; Bionute 62%, Fumus 68%, and Jetton 58%, compared to the no Brassica treatment, 82%, over both years. The highest rate of amendment had the greatest level of suppression and was significantly different from the lowest rate of application, with the medium rate or standard rate of production being more similar to the lowest rate. For crown ratings, percentage of plants with crown lesions was ½x rate 44%, 1x rate 42% and 3x rate 23%,over both years. Similarly, the amendments reduced percent root discoloration ½x rate 27%, 1x rate 25% and 3x rate 19%, over both years. Isolation of Rhizoctonia solani from plants was also reduced; ½x rate 71%, 1x rate 65% and 3x rate 56%, over both years. Plant growth of petunias and impatiens was not affected by the Brassica amendment treatments or the rate of application.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

It anticipated that a refereed publication will come out of this research upon completion of the thesis.

Project Outcomes

Project outcomes:

This study indicates that Brassica selection has less effect than the amount of Brassica amendment incorporated. Rate of application was observed to have the greatest impact on effective disease suppression on both impatiens and petunias in soil infested with Rhizoctonia solani. Thus, Brassica selection should be based on biomass production and not glucosinolate levels produced by a cultivar. It other experiments conducted, the suppressive effect seemed to be evident from incorporation for as long as 14 days after incorporation for experiments with Thielaviopsis basicola, Pythium aphanidermatum and Rhizoctonia solani indicating that some mechanism of action other than the formation of biocidal quantities of volatile compounds is responsible for pathogen suppression. These results help growers because they can make selections based on adaptation of the Brassica species and cultivar. Also this allows cost savings by purchasing lower cost cultivars. The study did document the suppression of the fungus Rhizoctonia solani on ornamentals, but results also should apply to vegetable crops where Rhizoctonia solani is an important pathogen. The work with the root-knot nematode was inconclusive primarily due to methodology. Galling from the root-knot nematode on cotton has been significantly reduced using Fumus to levels similar to the fumigant Telone II.

Economic Analysis

Use of Brassica cover crops may be the only viable option for growers for the control of soilborne pathogens that require fumigation. Other options such as crop rotation out of a susceptible crop for several years are often not economical or practical with growers that have limited land for production. Estimated costs for seed would be approximately $10 to $20 per acre based on the seeding rates used for biomass production in field studies. This is substantially cheaper that fumigant nematicides or general fumigants.

Farmer Adoption

This strategy was design to be a scale neutral disease control strategy for use by large and small scale producers, as well as homeowners. Increased testing in production systems should be examined prior to making general production recommendations.

Recommendations:

Areas needing additional study

Large scale studies in growers’ fields need to be conducted to validate the findings with selected pathogens. Field studies would examine the effects of Brassica amendments on multiple pathogens for a particular crop and look at the economic returns from the control of these pathogens.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.